1. Why do nanoparticles need capping agent? 2. Why polymers like PVP can be used to stabilise nanoparticles?

Capping agents stabilizes the nanoparticles and prevents them from aggregating. Polymers are good stabilizing agents because they can cover a larger surface area of the nanoparticles. Other organic agents, which gives the nanoparticles different functionalities. OTher organic agents can be used though as long as it can attach itself with the nanoparticles.